Electron multipliers



Apnl 7, 1959 J. J. PoLKosKY ET AL 2,881,343

' ELECTRON MULTIPLIERS Filed April 1e. 1957 INVENTORS JDSEPH J.PDLKDSKYE- AUGUST M. SPLINTER United btates Patent ELECTRON MULTIPLIERSJoseph J. Polkosky and August M. Splinter, Lancaster,

Pa., assignors to Radio 4Corporation of America, a corporation ofDelaware Application April 16, 1957, Serial No. 653,204

4 Claims. (Cl. 313-104) This invention relates to the art of makingsecondary electron-emitting electrodes and electron multiplier tubescontaining the same.

In low-voltage, high gain electron multipliers (e.g. photomultipliers,used as scintillation counters, headlight dimmers, etc.) it is usual toemploy dynodes made of nickel.

The principal object of the present invention is to provide an improvednickel dynode, and one characterized by its stability, long life and theuniformity of its electron emission over its surface.

The foregoing and related objects are achieved in accordance with theinvention by subjecting the target surface of a nickel dynode, orassembly of dynodes, to an oxidation process and subsequently depositingantimony and cesium on the nickel oxide.

The invention is described in greater detail by reference to the drawingwherein:

Fig. 1 is an elevational view, partly in section of -a photomultiplierembodying the invention; and,

Fig. 2 is a ow chart of the method of the invention.

The principles of the invention may be employed to prepare an electronmultiplier tube of the type known as the RCA 6655 photomultiplier. Sucha tube is shown in Fig. 1 and includes a tubular glass envelope 12having a transparent face plate 14 at one end on which is formed anelectron-emissive layer or photocathode 16 adapted to emit electronswhen exposed to light. The photocathode 16 may be of thealikali-antimony type or of any other suitable type and may be formed byevaporation of suitable substances onto the surface of the face plate14. A conductive lm 18 of aluminum, silver or the like, extends alongthe inner wall of the envelope 12 from the photocathode 16 toward theopposite end of the envelope and serves to allow electrical contact tobe made to the photocathode.

Spaced from the photocathode 16 is an apertured disk electrode 20 whichmay be employed as a focusing or accelerating electrode for electronsemitted by the photocathode. An electron multiplier assembly 22,comprising a plurality of secondary electron emitting electrodes ordynodes 24, is mounted directly behind the disk electrode 20 in such aposition as to receive photoelectrons passing therethrough. The electronmultiplier assembly 22 may be one of the type shown in U. S. Patent2,285,126 of Rajchman et al. The electrical constituents of the tube 10are suitably connected to electrical leads 26 which extend through aglass press 27 which is sealed to the balance of the envelope. The press27 includes an exhaust tubulation 29. The materials from which thephotocathode is prepared are provided within the envelope in suitableform and suitably supported for evaporation onto the face plate of theenvelope. Such materials may be provided as pellets 28 and 30 mounted onsuitable heatable wires 32 and 34 or the like secured to the apertureddisk and facing the face plate 14. A metal container 36 suitable forcarrying materials from which cesi- Patented Apr. 7, 1959 ice um vapormay be obtained is suitably mounted within the tube, for example on oneof the leads 26.

The dynodes 24 of the electron multiplier assembly 22 are made of nickeland in preparing the multiplier assembly the dynodes are cut from asuitable strip of nickel and are suitably formed, shaped and cleaned.The dynodes thus assembled are then placed in a suitable container andvacuum baked in an oven at a temperature in the range of 800 C. to 900C. for a time of the order of 15 to 40 minutes. This operation isperformed to degas the dynode assembly and, ordinarily, baking for aboutone-half an hour at 800 C. is satisfactory. Next, oxygen is admitted tothe oven at a pressure of about one inch of mercury and baking iscontinued at about 800 C. for about one minute. This operation providesa coating of nickel oxide on the dynodes.

The electron multiplier assembly, having oxidized nickel dynodes, isthen removed from the oven but is maintained in the oxygen atmosphereand allowed to cool to room temperature. When the assembly has cooled toroom temperature, a layer of antimony is formed by evaporation on thenickel oxide surfaces of the dynodes according to conventional practice.

T he dynodes are then assembled to form the multiplier assembly 22having dynodes of nickel oxide coated with antimony. The multiplierassembly is then mounted on the press 27 and the press is sealed to thetube envelope 12. The envelope is evacuated, the photocathode 16 isprepared in known fashion and then the tube is baked to degas all parts.Baking for one-half to two hours at a temperature in the range of 300 C.to 400 C. is satisfactory. Finally, vapor of an alkali metal, such ascesium, is released in the envelope from the container 36, which isheated by induced radio frequency currents. The cesium vapor reactingwith the dynode surfaces completes their activation.

Electron multi-pliers having dynodes made of nickel and processedaccording to the invention have been found to provide output currents orgains of the order of 3000 microamperes with 10-5 lumens of light on thephotocathode and with volts applied to each dynode.

What is claimed is:

l. An electron multiplier electrode comprising a base vof nickel, acoating of nickel oxide on said base, and antimony and an alkali metalpresent on said nickel oxide coating.

2. An electron multiplier electrode comprising a base of nickel having acoating of nickel oxide, said nickel oxide coating including reactionproducts of antimony and an alkali metal.

3. An electron multiplier electrode comprising a base of nickel having acoating of nickel oxide, said nickel oxide coating including reactionproducts of antimony and cesium.

4. An electron multiplier tube comprising an envelope and containing anelectron multiplier assembly made up of electron multiplier electrodescomprising a nickel ox ide layer on a base of nickel, said nickel oxidelayer containing reaction products of antimony and cesium.

References Cited in the tile of this patent UNITED STATES PATENTS2,620,287 Bramley Dec. 2, 1952 FOREIGN PATENTS 58,988 Denmark Aug. 18,1941 OTHER REFERENCES Physics and Applications of Secondary ElectronEmission, Bruining, McGraw-Hill, 1954, QC 721 B86.

1. AN ELECTRON MULTIPLIER ELECTRODE COMPRISING A BASE OF NICKEL, ACOATING OF NICKEL OXIDE ON SAID BASE, AND ANTIMONY AND AN ALKALI METALPRESENT ON SAID NICKEL OXIDE COATING.